A semiconductor device includes a substrate, a first source/drain structure, a vertical channel layer, a gate structure, a second source/drain structure and a body epitaxial layer. The first source/drain structure is over the substrate. The vertical channel layer is over the first source/drain structure. The gate structure is on a first sidewall of the vertical channel layer. The second source/drain structure is over the vertical channel layer. The body epitaxial layer is on a second sidewall of the vertical channel layer. The body epitaxial layer and the vertical channel layer are of opposite conductivity types, and the body epitaxial layer is separated from the gate structure.